hbrobjerg

The issue was the theme “Dante by Swift Ideas.”

This theme adds many “Meta options” to the editor as seen here:

However, some of these metadata require to be set for certain functions to work. In this case, “sf_detail_type” must be set to “image” for the featured image to show in posts.

Hence, I added ‘meta’ => $metaData, where $metaData[‘sf_detail_type’] = ‘image’;

For this to work with REST API, I had to build a custom plugin to add this metadatum to be accepted:

<?php
/*
Plugin Name: Add meta tags to REST-API
Description: Adds the specified meta tags to the REST-API for supporting posting via same.
Version: 1.0
Author: None
*/

// If this file is called directly, abort.
if ( ! defined( 'WPINC' ) ) {
die;
}

function register_my_meta() {
register_meta('post', 'sf_detail_type', array(
'show_in_rest' => true,
'single' => true,
'type' => 'string',
'auth_callback' => function() {
return current_user_can('edit_posts');
}
));
}
add_action('init', 'register_my_meta');

Posting now works and the issue has been resolved.